Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology in which the data is stored in a magnetic storage element that makes up the memory bit cell. The magnetic storage element is a ferromagnetic layer (called the “free layer”) in a multilayer pillar structure that forms a resistive element connected to a conventional complementary metal-oxide-semiconductor (CMOS) or selector device in an individual bit cell of a memory array. The magnetic orientation of the free layer is typically constrained to align along a particular axis and its direction along that axis (e.g., up or down) defines the binary state of the data storage. In perpendicular MRAM devices, this axis is parallel to the long axis of the pillar and perpendicular to the plane of the individual layers.
The magnetic orientation of the free layer is measured with respect to another ferromagnetic layer (the “reference layer”) that has a magnetic orientation strongly pinned in one of the directions along the axis (e.g., up). The free and reference layers are separated by a non-magnetic spacer layer. In the most common application the spacer layer is a thin oxide insulator such as MgO and the free and reference layers are ferromagnetic metals, forming a tri-layer structure called a magnetic tunnel junction (MTJ).
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel magnetic alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized electron tunneling effect, the electrical resistance of the cell depends on the relative orientation of the magnetizations of the two layers. The bit cell resistance is therefore different for the parallel and anti-parallel states and thus the cell resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. However at high memory array densities the bit cells become quite small so the magnetic anisotropy of the free layer and reference layer needs to be large enough to withstand thermal fluctuations.
Spin transfer torque or spin transfer switching, uses spin-aligned (“polarized”) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction. In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer (i.e., polarizer), thus producing a spin-polarized current. If a spin-polarized current flows into the free layer in the magnetic tunnel junction device, the electrons will transfer a portion of their spin-angular momentum to the free layer thereby producing a torque on the magnetization of the free layer. Thus, this spin transfer torque can switch the magnetization of the free layer, which, in effect, writes either a “1” or a “0” based on whether the free layer is in the parallel or anti-parallel state relative to the reference layer.
When a current is passed through a magnetic layer (e.g., a polarizer), the spin orientation of the electrons that flow out of the magnetic layer is generally aligned in the direction of the magnetization of the magnetic layer and will exert a spin-transfer torque in that direction (forming a transverse spin current) upon injection into another magnetic layer. However, due to the conservation of angular moment for the system, the electrons on the opposite side of magnetic layer, those that do not go through the magnetic layer, generally have a spin orientation that is aligned in the direction that is anti-parallel to the magnetization direction of the magnetic layer. The net effect of this process is that the current applied to the magnetic layer undergoes spin filtering, which creates a spin current on one side of the magnetic layer, with spins that are aligned with magnetization direction of the magnetic layer, and a reflected spin current on the other side of the magnetic layer, with spins that are anti-parallel to the magnetization direction of the magnetic layer. This effect occurs upon application of a current to any magnetic layer, including an in-plane polarization layer or an out-of-plane reference magnetic layer. Thus, in a typical MTJ, when switching the magnetization direction of the free layer in one direction (e.g., from the parallel to anti-parallel state) is achieved using spin transfer torque from the transverse spin current, switching the free layer in the other direction (e.g., from the anti-parallel to parallel states) would be achieved using spin transfer torque from the reflected spin current. This is typically accomplished by running electrical current through the MTJ in one direction when switching from the anti-parallel to parallel state and running the electrical current through the MTJ in the other direction when switching from the parallel to anti-parallel state.
FIG. 1 illustrates a magnetic tunnel junction (“MTJ”) stack 100 for a conventional MRAM device. As shown, stack 100 includes one or more seed layers 110 provided at the bottom of stack 100 to initiate a desired crystalline growth in the above-deposited layers. Furthermore, MTJ 130 is deposited on top of SAF layer 120. MTJ 130 includes reference layer 132, which is a magnetic layer, a non-magnetic tunneling barrier layer (i.e., the insulator) 134, and the free layer 136, which is also a magnetic layer. It should be understood that reference layer 132 is also part of SAF layer 120. As shown in FIG. 1, magnetic reference layer 132 has a magnetization direction perpendicular to its plane. As also seen in FIG. 1, free layer 136 also has a magnetization direction perpendicular to its plane, but its direction can vary by 180 degrees.
The first magnetic layer 114 in the SAF layer 120 is disposed over seed layer 110. SAF layer 120 also has an antiferromagnetic coupling layer 116 disposed over the first magnetic layer 114. Furthermore, a nonmagnetic spacer 140 is disposed on top of MTJ 130 and a polarizer 150 is disposed on top of the nonmagnetic spacer 140. Polarizer 150 is a magnetic layer that has a magnetic direction in its plane, but is perpendicular to the magnetic direction of the reference layer 132 and free layer 136. Polarizer 150 is provided to polarize a current of electrons (“spin-aligned electrons”) applied to MTJ structure 100. Further, one or more capping layers 160 can be provided on top of polarizer 150 to protect the layers below on MTJ stack 100. Finally, a hard mask 170 is deposited over capping layers 160 and is provided to pattern the underlying layers of the MTJ structure 100, using a reactive ion etch (RIE) process.
In all prior MTJ devices using a polarizer such as polarizer 150, the magnetization direction of polarizer 150 is fixed, which is shown in FIGS. 1 and 3. See also U.S. Pat. No. 6,532,164, which states that the direction of the magnetization of the polarizing layer cannot vary in the presence of current. Prior to current passing through the MTJ, the free layer 136 has a magnetization direction 200 perpendicular to that of the polarizer 150. While the magnetization direction 200 of the free layer 136 can rotate by 180 degrees, such rotation is normally precluded by the free layer's inherent damping ability 205, which is represented by a vector 205 pointing to axis 202 (shown as a dashed line in FIG. 2a as well as FIG. 3). Axis 202 is perpendicular to the plane of free layer 136. This damping 205 has value, defined by the damping constant, which maintains the magnetization direction of the free layer 136.
Passing a current through polarizer 150 produces a spin-polarized current, which creates a spin transfer torque 210 in the direction of the polarizer 150 on the magnetization vector 200. This spin transfer torque from the polarizer adds to the main spin transfer torque that causes free layer magnetization direction switching. In devices like those shown in FIG. 1, when the spin transfer torque 210 begins to help overcome the damping 205 inherent to the free layer 136, the magnetic direction 200′ begins to precess about its axis, as shown in FIG. 2a. As seen in FIG. 3, spin transfer torque 210 helps the magnetization direction of the free layer 136 to precess in a cone-like manner around an axis 202 perpendicular to the plane of the layers. When a spin polarized current traverses the stack 100, the magnetization of the free layer 136 precesses in a continuous manner (i.e., it turns on itself in a continuous manner as shown in FIG. 3) with maintained oscillations until the magnetic direction of free layer 136 is opposite the magnetic direction prior to the spin torque causing precession, i.e., the magnetic direction of free layer 136 switches by 180 degrees.
In the absence of polarizer 150, random thermal events are required to induce precession of the free layer. In such simple perpendicular MTJ devices, the spin current is only generated by the reference layer 132. Therefore, the spin-polarized electrons generally have a direction that is perpendicular to the plane of the free layer 136 (i.e., aligned parallel with the magnetization vector of the reference layer 132 when the transverse spin current is used to switch the free layer 136 and aligned anti-parallel to the magnetization vector of the reference layer 132 when the reflected spin current is used to switch the free layer 136). However, because both stable directions of the magnetization vector of the free layer 136 are also perpendicular to the plane, spin-polarized electrons with such a perpendicular alignment exert no net spin transfer torque on the free layer 136. Only when the magnetization vector of free layer 136 deviates from a purely perpendicular orientation can the spin current exert a spin transfer torque on the free layer 136. Therefore, in simple perpendicular MTJ systems that lack an in-plane polarizer, random thermal events are required to shift the magnetization vector of the free layer 136 off of the perpendicular axis so that the spin-polarized current generated by the reference layer 132 can exert a spin transfer torque on the free layer 136, thereby switching it from the first magnetization direction to the second.
The use of an in-plane polarizer, such as polarizer 150, can enhance the efficiency of switching free layer 136 by initiating the precession of free layer 136. FIG. 3 illustrates precession of free layer 136 of an MTJ assisted by spin polarized current provided by polarizing magnetic layer 150. The spin polarized electrons from polarizer 150 provide torque 210 that helps overcome the damping 205 in the first half of the precession 215 because the torque 210 provided by the spin polarized current is opposite that of the inherent damping 205 of the free layer 136. This is shown on the right-hand side of the middle portion of FIG. 3. Thus, in-plane polarizer 150 allows the generation of instant spin transfer torque on the free layer 136 upon the application of an electrical current to the MTJ stack 100. The in-plane polarizer contributes an in-plane (i.e., orthogonal) spin torque that can immediately act on the magnetization vector of the free layer, opposing damping 205 and pulling the vector 200 off of the perpendicular plane. This, in turn, allows the perpendicular component of the spin transfer torque to act on the magnetization vector of the free layer 136, thereby obviating the need for the random thermal event described above. In this way, the orthogonal polarizer can enhance the efficiency of switching the free layer.
However, while the use of a perpendicular MTJ with an orthogonal polarizer may increase the efficiency of switching the free layer, such structures might also suffer from a concomitant reduction in thermal stability. Particularly in smaller devices, the effective magnetization of the free layer is reduced, thereby allowing switching of the free layer to occur at the low currents needed for commercial applicability. The presence of an orthogonal polarizer can further destabilize the free magnetic layer due to magnetic and/or electronic effects potentially leading to an increased probability for random, unintended switching, particularly during application of the read current to the device (i.e., read disturb). This higher probability of random switching and read disturb can limit the commercial applicability of the device, leading to a shorter duration of memory retention and reduced thermal stability.
Thus, in prior devices, because magnetization direction of in-plane polarizer 150 is fixed, structures utilizing such a polarizer suffer from an increased probability of read disturb or other unintended switching of the free layer. This, in turn, reduces the commercial applicability of such devices due to reduced durations of memory storage and impaired thermal stability. Such deleterious attributions are in part the result of the fact that the polarizer 150 continues to destabilize the free layer in the absence of the electrical current used to write the bit (i.e., the programming current).
Thus, there is a need for a spin torque transfer device that reduces the amount of current needed for switching while also switching at high speeds and requiring reduced chip area. Such device should also be stable during the when reading the bit and during periods of inactivity.